Add DWARF index cache

Add DWARF index cache

New in v3:

- Remove things related to the dwarf-5 format.
- Fix compilation on mingw (scoped_mmap.c).

GDB can generate indexes for DWARF debug information, which, when
integrated in the original binary, can speed up loading object files.
This can be done using the gdb-add-index script or directly by the
linker itself.  However, not many people know about this.  And even
among those who do, because it requires additional steps, I don't know a
lot of people who actually go through that trouble.

To help make using the DWARF index more transparent, this patch
introduces a DWARF index cache.  When enabled, loading an index-less
binary in GDB will automatically save an index file in ~/.cache/gdb.
When loading that same object file again, the index file will be looked
up and used to load the DWARF index.  You therefore get the benefit of
the DWARF index without having to do additional manual steps or
modifying your build system.  When an index section is already present
in the file, GDB will prefer that one over looking up the cache.

When doing my edit-compile-debug cycle, I often debug multiple times the
same build, so the cache helps reducing the load time of the debug
sessions after the first one.

- The saved index file is exactly the same as the output of the "save
  gdb-index" command.  It is therefore the exact same content that would
  be found in the .gdb_index or .debug_names section.  We just leave it
  as a standalone file instead of merging it in the binary.

- The cache is just a directory with files named after the object
  file's build-id.  It is not possible to save/load the index for an
  object file without build-id in the cache.

- The cache uses the gdb index format.  The problem with the dwarf-5
  format is that we can generate an addendum to the .debug_str section
  that you're supposed to integrate to the original binary.  This
  complicates a little bit loading the data from the cached index files,
  so I would leave this for later.

- The size taken up by ~/.cache/gdb is not limited.  I was thinking we
  could add configurable limit (like ccache does), but that would come
  after.  Also, maybe a command to flush the cache.

- The cache is disabled by default.  I think once it's been out there
  and tested for a while, it could be turned on by default, so that
  everybody can enjoy it.

- The code was made to follow the XDG specification: if the
  XDG_CACHE_HOME environment variable, it is used, otherwise it falls
  back to ~/.cache/gdb.  It is possible to change it using "set
  index-cache directory".  On other OSes than GNU/Linux, ~/.cache may
  not be the best place to put such data.  On macOS it should probably
  default to ~/Library/Caches/...  On Windows, %LocalAppData%/...  I
  don't intend to do this part, but further patches are welcome.

- I think that we need to be careful that multiple instances of GDB
  don't interfere with each other (not far fetched at all if you run GDB
  in some automated script) and the cache is always coherent (either the
  file is not found, or it is found and entirely valid).  Writing the
  file directly to its final location seems like a recipe for failure.
  One GDB could read a file in the index while it is being written by
  another GDB.  To mitigate this, I made write_psymtabs_to_index write
  to temporary files and rename them once it's done.  Two GDB instances
  writing the index for the same file should not step on each other's
  toes (the last file to be renamed will stay).  A GDB looking up a file
  will only see a complete file or no file.  Also, if GDB crashes while
  generating the index file, it will leave a work-in-progress file, but
  it won't be picked up by other instances looking up in the cache.

gdb/ChangeLog:

	* common/pathstuff.h (get_standard_cache_dir): New.
	* common/pathstuff.c (get_standard_cache_dir): New.
	* build-id.h (build_id_to_string): New.
	* dwarf-index-common.h (INDEX4_SUFFIX, INDEX5_SUFFIX,
	DEBUG_STR_SUFFIX): Move to here.
	* dwarf-index-write.c (INDEX4_SUFFIX, INDEX5_SUFFIX,
	DEBUG_STR_SUFFIX): Move from there.
	(write_psymtabs_to_index): Make non-static, add basename
	parameter.  Write to temporary files, rename when done.
	(save_gdb_index_command): Adjust call to
	write_psymtabs_to_index.
	* dwarf2read.h (dwarf2_per_objfile) <index_cache_res>: New
	field.
	* dwarf2read.c (dwz_file) <index_cache_res>: New field.
	(get_gdb_index_contents_from_cache): New.
	(get_gdb_index_contents_from_cache_dwz): New.
	(dwarf2_initialize_objfile): Read index from cache.
	(dwarf2_build_psymtabs): Save to index.
	* dwarf-index-cache.h: New file.
	* dwarf-index-cache.c: New file.
	* dwarf-index-write.h: New file.

gdb/testsuite/ChangeLog:

	* boards/index-cache-gdb.exp: New file.
	* gdb.dwarf2/index-cache.exp: New file.
	* gdb.dwarf2/index-cache.c: New file.
	* gdb.base/maint.exp: Check if we are using the index cache.